1. Field of the Invention
The present invention relates to a variable venturi type carburetor having a suction piston adapted to move into and out of a suction chamber in accordance with the change of flow rate of the intake air so as to change the cross-sectional area of the venturi section defined at the upstream side of a throttle valve, and to meter the fuel to be injected through a cooperation of a metering needle attached to the head thereof with a metering jet.
More particularly, the invention is concerned with a variable venturi type carburetor in which the side of the suction piston head opposing to the throttle valve is cut and removed obliquely to avoid the attaching of fuel. Still more particularly, the invention is concerned with a variable venturi type carburetor of the kind stated above, wherein the throttle valve is so pivotally mounted that it can be fully opened without interference with the obliquely cut head of the suction piston, thereby to prevent the fuel from attaching to the inner peripheral surface of the throttle bore while reducing the whole height of the carburetor.
2. Description of the Prior Art
It is well known that carburetors for automobile engines or the like can be broadly sorted into a stationary or fixed venturi type carburetors and variable venturi type carburetors. In general, the latter exhibit various superiority or advantages over the former.
For instance, the variable venturi type carburetor has a simple construction. Namely, as shown in FIG. 1, the variable venturi type carburetor has a suction piston 2 adapted to be moved into and out of a suction chamber 1 in response to the change in the flow rate of the intake air so as to change the cross-sectional area of a venturi section 3 of the carburetor. At the same time, a metering needle 5 extended from the head 4 of the carburetor cooperates with a metering jet 6 in metering the fuel to be injected. Namely, this type of carburetor can meter only at first position, without any non-linearity or discontinuity of the metering characteristic. Therefore, this type of carburetor inherently has a superior transient response characteristic to that of the fixed or stationary venturi type carburetor. Thus, this type of carburetor is quite advantageous and preferred from the view point of current demand for pollution prevention.
There are, however, various drawbacks or shortcomings even in this variable venturi type carburetor, one of which is the unstable idling operation.
More specifically, in the variable venturi type carburetor, the throttle bore 7 of the single barrel is made to have a diameter large enough to effectively maintain a sufficiently large engine output under the condition of full-throttle opening. Therefore, in the operation state in which the fuel consumption is extremely small, i.e. when the flow rate of intake air is small as in the case of idling, the fuel coming from the metering jet 6 is not effectively suspended by the air in the throttle bore. As a result, the mixing of the fuel with air is rendered insufficient and incomplete so that the fuel flows assuming a form of a liquid fuel film A in contact with the inner surface of the barrel 8.
Then, the fuel attaching to the inner surface is kept in the attaching state, i.e. no atomizing energy is imparted to the liquid fuel attaching to the inner surface, due to a turbulency of the air flowing through the throttle barrel. Thus, the fuel forms a plurality of fuel pools B on the inner surface of the throttle bore 7 and falls directly onto the upper end surface of the throttle valve 9.
Meanwhile, the fuel scraped off from the inner surface by the head 4 of the suction piston 2 is accumulated on the lower edge 10 of the end of the suction piston head 4 to form a fuel pool C which also drops onto the upper end surface of the throttle valve 9.
These drops of fuel inconveniently cause uneven or not uniform distribution of fuel on the upper surface of the throttle valve 9. In addition, the distribution itself is unstable. In consequence, the flow of the mixture is unstabilized, even when the intake vacuum is high to generate a subsonic flow of intake air passing the edge of the throttle valve 9, as illustrated. This phenomenon has been confirmed through an experiment conducted employing a model of a carburetor made of a transparent plastic material.
As a result, as shown in FIG. 4a, the air-fuel ratio of the mixture is rendered unstable and, in case of a multi-cylinder engine, the distribution of the mixture over the all cylinders is rendered not uniform to seriously deteriorate the stability of the engine operation.
The problem of unstability of idling operation is important and vital. In order to overcome this problem, there have been take various countermeasures. One of these countermeasures is to set the idling speed of the engine at a relatively high level. This countermeasure, however, is not recommended because it inevitably deteriorates the fuel consumption particularly in the low-speed city running mode including repeated start and stop at crossings.
As an alternative countermeasure, it has been proposed to provide a slow passage separately so as to stabilize the idling operation. Although this countermeasure can provide a stabilized idling operation of the engine, it poses, on the other hand, a problem of undesirable so-called "discontinuity" or "non-linearity" of the characteristic in the transient region between the slow and main regions, due to the fact that the slow passage and the main passage are provided separately from each other, resulting in a deteriorated air-fuel ratio characteristic in the transient region. This seriously degrades and spoil the aforementioned advantage of the variable venturi type carburetor, i.e. the superior transient characteristic.